1. Field of the Invention
The invention relates to a tracking servo apparatus of an optical information recording and reproducing apparatus for recording and reproducing information to/from an optical disc.
2. Description of the Related Art
In an apparatus for recording or reproducing information to/from an optical disc, if a recording layer surface of the optical disc is directly exposed, the exposure causes problems. When the surface is scratched or damaged by a collision with a head, the recording or reproduction cannot be performed. The writing or reading, therefore, is performed through a transparent layer of a predetermined thickness. Although the transparent layer is formed by, for example, injection molding, it is difficult to accurately set a thickness of the transparent layer to a specified value for the whole surface. Usually, a thickness error of several tens of μm occurs.
The thickness error of the transparent layer causes the occurrence of a spherical aberration. Due to the occurrence of the spherical aberration, a salient deviation is caused between a focusing offset value which gives a maximum amplitude value of an RF signal as an information read signal and a focusing offset value which gives a maximum amplitude value of a tracking error signal. If a focusing offset value is, therefore, adjusted so as to maximize an amplitude of the tracking error signal, there is a problem such that a level of the maximum amplitude value of the RF signal decreases.